The present invention relates to a surface finishing or processing method for a stainless steel material, and a manufacturing method for a metal gasket, such as a gasket, for an exhaust manifold for an engine, or a cylinder head gasket and the like.
When a joint surface between an exhaust manifold and an exhaust pipe for an automobile engine, or a joint surface between a cylinder head and a cylinder block (cylinder body) is sealed, a metal gasket is clamped between members to seal combustion gas, coolant water, and lubrication oil.
Such a metal gasket is generally manufactured by the following method. At first, after a metal plate made by soft steel, annealed stainless (annealed material), or stainless steel material (spring steel) is cleaned and a general surface finishing such as a chromate filming is applied, a primer is applied to the metal plate. And then, the metal plate is formed in the shape of a metal gasket by punching with a press die corresponding to each metal gasket, i.e. cutting to the gasket shape. This metal formation plate is pressed, and a seal bore is opened, or a depression and a projection such as a folding back portion (grommet), a full bead, or a half bead, are formed. Next, when a middle plate is used, rubber or resin is applied to the middle plate according to necessity. When the metal formation plates are ready, these metal formation plates are assembled to form a metal gasket. The front and the back surfaces of this assembled metal gasket are coated with rubber or resin. After the coating is completed, the metal gasket is inspected. After the approval of the inspection, the metal gasket is shipped as a product.
However, in the conventional art, the surface of the metal plate including the metal gasket is finished after being cleaned by organic solvent, alkali, acid, or water type cleaner. Accordingly, liquid waste disposal, or a technology or equipment for preventing the release of volatile organic compounds (VOC) contained in the organic solvent into environment, is required. The volatile organic compounds are considered as one of causative agents of photochemical oxidant and suspended particulate matter, and reducing volatile organic compounds has been strongly required from the standpoint of environmental conservation.
On the other hand, a water type paint in which the organic solvent in the paint is replaced by water has been developed and used for the surface of various types of metal plates. Regarding the application of the water type paint, the following coating method of a molding material, such as shown in Japanese Patent Publications No. 11-156274, is proposed. The coating method of the molding material comprises a cleaning process wherein the surface of the molding material is cleaned by spraying water into the surface; a water removal process wherein water attached on the surface of the molding material is removed; a paint application process wherein water type paint containing alcohol is applied to the molding material; and a high-speed hot air type drying process wherein hot air is sprayed at high speed on the surface of the molding material on which the paint is applied.
However, in the cleaning process of the coating method, in order to remove molding oil, metal ion, dust and the like, high-pressure hot water cleaning equipment which sprays hot water at 60° C.˜70° C. from a hot water spray ring with a pressure at approximately 5˜7 kg/cm2, is used. Accordingly, the high-pressure hot water cleaning equipment must be large.
Also, when the surface of the stainless steel material is processed by a water type surface finishing which applies a water type surface finishing agent, such as a trivalent chromium, silica, and zirconia type, in order to improve adhesiveness of the primer, wetness 73 in the “Wetting Tension Test Method” of “JIS K 6768:1999” is required. In comparing with the wetness approximately 52 when a solvent type primer is applied without the water type surface finishing, specific wetness is required. The value of the wetness 73 relative to the water type surface finishing cannot be easily obtained. Enough wetness cannot be obtained by dry cleaning with a low-pressure mercury vapor lamp, or even if enough wetness can be obtained, a long processing time is required.
The metal gasket such as the cylinder head gasket has been used in a severe condition with an improvement of an engine, and also peeling of the paint, such as rubber or resin, has become a big problem. Therefore, the improvement of the adhesiveness of the primer has become an important issue, since the improvement of the adhesiveness of the primer' has a large effect on the prevention of the peeling of the paint which is going to be coated on the top of the primer.
The present invention has been made in order to solve the problem described above, and the object of the invention is to provide a surface finishing or processing method for the stainless steel material, and a manufacturing method for the metal gasket, which uses a relatively small equipment; does not require cleaning with an organic solvent, alkali, acid, or water type cleaner; and can make a great contribution to environmental conservation by not creating waste fluid and its disposal and by preventing the release of the volatile organic compounds.
Further objects and advantages of the invention will be apparent from the following description of the invention.